This invention relates to a new system referred to as "T-NET" which provides bidirectional communication of digital information to a plurality of fixed or mobile subscribers on a vacant TV channel adjacent to, and cooperating with an existing "Host" television (TV) station. The horizontal and vertical sync pulses of the host TV signal are used as a wide-area clock to coordinate time and frequency division multiplexing of subscriber transponders and to trigger up-link responses from them only during the horizontal blanking interval (HBI) to prevent interference to television viewers Down-link signals to subscribers are also sent within the HBI. In a preferred embodiment the typical subscriber-to-central receiver data rate is 300 or 1200 baud and that signal's spectrum is "spread" into subchannels 187.5 KHz wide by virtue of modulating it on a stream of 5 microsecond pulses. Thirty-two of these subchannels fit in a standard 6 MHz TV channel More than 300 transponders can operate simultaneously on each subchannel. The same subchannels may be used for up-link and down-link communications, even simultaneously on the same subchannel.
Means to multiplex information to TV receivers on an existing TV signal during its horizontal or vertical blanking interval are in use or have been contemplated (e.g. present day "Teletext"). However, the advantage of using TV horizontal and/or vertical sync pulses to synchronize both down-link and up-link radio signals, on the same or an adjacent TV channel, so they effectively exist only within TV horizontal or vertical blanking intervals, thus are invisible to television viewers, and for the further purpose of enabling time and frequency division multiplexing of many signals, has not heretofore been discovered. The present invention teaches that technology.
A major portion of the U.S. radio spectrum has been allocated to broadcast services and more specifically to television. A substantial number of television channels are unused in most cities because of physical limitations caused by inadequate television receiver selectivity. As a consequence of this at least one vacant channel exists between assigned television stations and those channels have heretofore been unusable. As a practical matter, intermodulation interference and other considerations further limit the number of usable television channels so that substantially less than half the allocated TV channels are in use in a given area. Unusable channels are sometimes referred to as "taboo" frequencies. A principal object of the instant invention is to make practical use of this presently unusable spectrum.
Described another way, typical television receivers, particularly when operating at UHF frequencies, have relatively poor frequency selectivity consequently radio transmission in channels adjacent to a TV signal is prohibited because it would cause unacceptable interference. For example, even a low power conventional radio device which transmits one watt could easily cause unacceptable interference to adjacent channel television viewers who live within a radius of several city blocks surrounding it because its power would overwhelm the TV signal. Clearly, thousands of such conventional transmitters deployed throughout a city for the uses contemplated here would generate unacceptable interference.
Since television broadcast channels are by government regulation allocated to "mass media" use, it is implicit that such channels are not intended for low capacity private radio communications such as associated with point-to-point or land mobile radio applications. Consequently applications for the aforementioned vacant TV channels, if they could be used at all, would be expected to benefit the public en masse as contemplated in the instant invention for such uses as future home information systems, interactive television, remote shopping, banking electronic mail, reservations means, security alarm communications, and the like.
Ease of installation and simplicity of operation are important considerations for mass applications. Thus sharing of the user's existing television antenna as taught here is a important feature. Integrating this invention with a television receiver to provide interactive TV controllable from remote hand-held devices comparable to those used today for remote TV channel switching are other features taught in this specification.
An object of the present invention is to provide means to accurately partition subscribers into geographic "radio cells" within which specific subscriber transponder subchannels may be assigned and isolated from transponders in other cells. This permits re-use of subchannel frequencies in non-contiguous cells to significantly expand the number of users that can operate on one previously vacant TV channel in a given city. These desirable frequency re-use features are commonly identified today with "Cellular Radio".
The instant invention is also applicable to two-way cable TV systems (CATV) to provide improved isolation of up-link and down-link signals compared to existing methods.
A further object of the present invention is to provide improved means for locating and tracking the position of mobile or portable subscriber transponders to provide economical services sometimes referred to as automatic vehicle location or automatic vehicle monitoring (AVM).
Automatic "hand-off" of present day cellular radio telephone subscribers as they move from cell-to-cell in a city is a problem because it is based on signal amplitude measurements and these vary widely at different places and at different times. An independent means such as T-NET to locate subscribers can form the basis for an alternative hand-off method which could minimize or solve the existing problem and this constitutes another T-NET application.
Yet another application of the invention is for so-called "video conferencing" which usually comprises a dedicated TV network connecting a central office with many remote offices for such applications as over-the-air teaching, presentations by management, or even TV monitoring of banks or other businesses for security alarm purposes. Such T-NET applications would employ the down-link to send pictures (video) and the up-link could either be digital or digitized "slow-voice", all multiplexed simultaneously with the existing TV program.
It is clear that simultaneous synchronization of the T-NET system with several TV stations in a city as contemplated by the inventor could be a problem. Thus a further object of the invention is to teach an operating means wherein the horizontal sync pulses of several co-located television transmitters are locked together in time so that subscriber transponders working in cooperation with one or several such stations will always transmit within the horizontal blanking interval (HBI) of all the television signals simultaneously, thus eliminating inteference to viewers of all of them. TV transmitter co-location is a practice in many large cities (e.g. Los Angeles and New York) to establish a common antenna direction for all TV viewers.
Alternatively, it is taught that if T-NET subscribers are located in a boundary service area between television stations not co-located (e.g. between TV transmitters in adjacent cities), then those subscriber transponders can be programmed to transmit only during the vertical blanking interval (VBI, which is much longer in time duration than the HBI) and thus will not interfere with TV viewers of either city, provided those television stations are synchronized to cause their vertical blanking intervals to overlap as taught in this invention.
Two new and improved methods are also taught for sending digital information to subscribers (down-link) by either: (1) co-channel modulation of the Host TV signal in a non-interferring manner or (2) modulating new "out-of-channel" subcarrier sidebands in adjacent upper or lower (or both) TV channels.